f(x)=f(x)sinx 相交于（2kπ+π/2, f(2kπ+π/2))[f(x)sinx]'=f'(x)sinx+f(x)cosx[f(2kπ+π/2)sin(2kπ+π/2)]'=f'(2kπ+π/2)sin(2kπ+π/2)+f(2kπ+π/2)cos(2kπ+π/2)=f'(2kπ+π/2)在交点处相切选C
